This invention relates to a method of and system for locating the beginnings of message segments on a copy recording duplicated from a master recording, and more particularly for correcting discrepancies in the message locations on the copy recording with respect to the message locations on the master recording introduced by the duplication process.
In one type of information retrieval system, prerecorded messages may be randomly accessed for being reproduced upon instruction. For example, an audio information retrieval system may include a magnetic tape cassette player for selectively reproducing audio messages which were prerecorded at specific physical locations along the cassette tape without an accompanying address cue or marker on the tape itself.
During recording of the audio messages on the cassette tape, the locations of the beginning of each message segment are either predetermined or contemporaneously determined through measurement of the tape lengths from the beginning of the tape ("BOT") to the start of each message segment. One measure of tape length is a count of the number of revolutions of one of the tape reels (either supply or take-up) between BOT and the beginning of a message segment. For example, the supply reel drive motor of the tape cassette transport may be monitored to generate a signal correlated to motor shaft rotation and the number of supply reel revolutions referenced to BOT. One type of useful signal includes a series of a predetermined number of electronic pulses for each revolution of the supply reel. During the recording process, therefore, the start of the first message segment will be located along the tape a first distance from BOT corresponding to a first pulse count from BOT, the start of the second message segment will be located along the tape a second distance from BOT corresponding to a second pulse count from BOT, the start of the third message segment along the tape will be located a third distance from BOT corresponding to a third pulse count from BOT, etc. In this manner, an index may be prepared correlating message segments (specified by respective message segment numbers) with respective pulse counts from BOT. This index or "look-up table" is available to a microcomputer controlling the player transport. In response to an operator's selecting a particular message for playback, the associated pulse count is " looked up" by the system's microprocessor and the pulses generated by the supply reel motor are monitored and tape movement is stopped (or slowed to normal playback speed) when this pulse count is reached.
If playback were performed using the original tape as recorded (i.e., the master recording), particularly played back on the same recorder/player transport, the above described method would locate selected messages for playback with accuracy. In many information retrieval applications, however, copy recordings are duplicated from the master recording, and the copies are distributed for being played back on player transports of variously located information retrieval systems. For example, an audio information retrieval system may be located in a passenger aircraft for providing keyboard access to prerecorded music and other passenger messages on a tape cassette. Duplicate cassettes are generally distributed to each aircraft, since it would be uneconomical and otherwise impractical to produce master recordings for distribution.
In duplicating copies from the master recording, the pulse counts from BOT to a particular message segment location associated with the copy may be different from the number of pulse counts associated with the location of the same message segment on the master. These discrepancies are caused during the duplication process primarily by the difference in tape speed produced by the transport reproducing the master tape and the transport recording the copy. Physical position errors in the locations of message segments are thereby introduced as the messages are recorded on the copy. The errors are cumulative from BOT as duplication proceeds and can be quite large toward the end of tape. For a 0.5% difference in the speeds of the two transports, the physical location of the last message segment on a sixty minute audio cassette may be erroneously displaced by more than fifteen inches. Accordingly, when retrieving messages from the copy tape recording, counting pulses from the look-up table associated with the master recording may position the copy tape at a place other than the actual beginning of the selected message segment.